underground conduits are ubiquitous in modern society. Conduits are used for carrying a wide variety of substances, including water, natural gas, oil, and sewage. In the past, these conduits have been typically comprised of plastics, concrete, or metals. One of the major problems with buried conduits comprised of concrete, concrete with metal reinforcements, or metal, is corrosion. The severity and rate of corrosion is dependent on the type of material comprising the conduit and the environment in which the conduit is buried. For example, ductile iron pipe (DIP) typically exhibits a low risk to severe corrosion compared to other metals. However, a rapid increase in the corrosion rate can be initiated by oxygenated water, tidal action, or specific soil types as soils containing sulfides. Because of high costs associated with removal and replacement of corroded conduits, the industry has expended substantial resources to solve this problem.
Initially, conduits were covered with paint coatings, wraps, or other materials to separate the conduit surfaces from the environment. Later, barrier films of polyethylene were used to protect DIP conduits. By insulating the exposed surfaces from soil, electrical currents, and oxygenated water, corrosion is usually prevented. However, due to improper installation, tears and punctures to the barrier film occurring during the installation and backfill process, free flow of water from tidal action, or soil or water becoming entrapped between the film and the conduit surface, actual corrosion still occurs in many cases. The industry has attempted to solve these problems by using more durable barrier films to encase the conduit surfaces, such as high density cross-laminated polyethylenes (HDCLPE). The superior impact strength, tear resistance, and tensile strength of HDCLPE has reduced some of the problems associated with the installation and backfill process; however, HDCLPE does not adequately address or control the problem of corrosion. Since there has not been an adequate alternative, present industry standards typically use either an 8 mil low density polyethylene (LDPE) film or a 4 mil HDCLPE film, a mil being equal to one thousandth of an inch (0.0254 millimeter), to wrap around the conduits for protection against corrosion.
Polyethylenes, as well as other plastic films, limit the free flow of water against the conduit surfaces, thereby reducing available oxygen. Any moisture which becomes trapped between the film and the conduit surface will eventually become deaerated. A problem arises where deaerated water levels are attained in the presence of sulfate reducing bacteria. Many anaerobic bacteria, such as Desulfovibrio desulfunicans, thrive in certain fresh water, brackish water, sea water, sulfate soils, or warm soil conditions. These bacteria act as a catalyst to initiate or augment the rate of corrosion in an environment that is normally adverse to corrosion. Additionally, other types of bacteria are believed to play a part in corrosion propagation and it appears that bacteria are also responsible for degradation of the polyethylene film. A possible solution to this problem is to treat the materials used to encase the conduit with bactericides. However, most bactericides are topical and water soluble, thereby offering only initial protection. Since conduits are buried for decades, this would not provide adequate long-term protection.
Another possible solution is to use certain volatile corrosion inhibitors (VCIs) which can be added to eliminate or reduce the presence of corrosion. An example of a commonly used VCI is illustrated in U.S. Pat. No. 3,425,954. These VCIs can be used to prevent conditions from developing inside the film barrier which are favorable to corrosion. VCIs work at a micron level to passivate the surface of metal with a passive film, thus reducing the chemical reactivity of its surface. VCIs are normally used in kraft papers for short term protection of metal parts, as illustrated in U.S. Pat. No. 4,557,966; however, paper is not suitable to be buried. VCIs could be added to the polyethylene film, but the effectiveness will be shortened since the vapor tends to escape from the film, thus preventing extended protection.
From the foregoing it may be seen that a need exists for an improved anti-corrosive material for protecting conduits buried in conditions favorable to corrosion.